Discharge lamps, such as cold cathode fluorescent lamps (CCFL) and external electrode fluorescent lamps (EEFL), are widely used to backlight liquid crystal displays (LCD). Until the CCFL is struck or ignited, it will not conduct current with an applied terminal voltage that is less than a striking voltage, e.g., the terminal voltage must be equal to or greater than the striking voltage (for example, 1500 Volts). Once an electrical arc is struck inside the CCFL, the terminal voltage may fall to an operating voltage that is approximately ⅓ of the striking voltage over a relatively wide range of input current. A driving apparatus is needed to provide an alternating current (AC) driving voltage and stable lamp current at a high frequency for the CCFL. Generally, the driving apparatus comprises a control circuit, a switching circuit, a transformer and a resonant circuit.
As LCD's have increased in size and popularity, multiple discharge lamps are needed as a backlight. FIG. 1 is a block diagram of a prior art apparatus for driving multiple parallel discharge lamps, comprising a switching circuit 101, a control circuit 102, a transformer 103, a resonant circuit 104 and a load 105. The load 105 comprises n parallel discharge lamps, and n capacitors each of which is serially connected to one discharge lamp to balance the lamp current, wherein n is a positive integer. This current balance technology is called a capacitor balance, and the capacitor is called a ballast capacitor. In normal operation, the terminal voltage of each discharge lamp is the voltage across the ballast capacitor subtracted from the output voltage Vout of the driving apparatus. Under an open lamp condition, since there is no current flowing through the open discharge lamp, the terminal voltage is exactly the output voltage Vout of the driving apparatus.
In FIG. 1, the ballast capacitors and discharge lamps constitute the load 105 of the driving apparatus. The load characteristic of the driving apparatus is quite different from that of a conventional driving apparatus without a capacitor balance. In a conventional driving apparatus without a capacitor balance, under a certain switching frequency, the output voltage of the driving apparatus will increase under open lamp condition. The more open lamps there are, the larger the output voltage. However, in the driving apparatus as shown in FIG. 1, under certain switching frequencies, the output voltage of the driving apparatus will decrease under an open lamp condition. The more open lamps there are, the smaller the output voltage. FIG. 2 illustrates the output voltage of the driving apparatus shown in FIG. 1 under different open lamp conditions, wherein H0(f) is the curve in normal operation, H1(f) is the curve with one open lamp, H2(f) is the curve with two open lamps, . . . , and Hn(f) is the curve with n open lamps. The output voltage Vout will decrease under open lamp condition and won't be high enough to ignite the lamp. Under certain switching frequencies, the output voltage Vout will decrease along with the increase of the number of the open lamps.
There are two prior open lamp protection methods. One is shutting down the driving apparatus once the open lamp condition is detected. This method cannot ignite the lamp again when the lamp is extinct. The other is setting the switching frequency of the driving apparatus to a predetermined frequency that is larger than the operation frequency (the switching frequency in normal operation), so as to increase the output voltage. But in the apparatus for driving multiple parallel discharge lamps with capacitor balance, since the load characteristics with different numbers of open lamps are different, the predetermined frequency is hard to be configured to fulfill all possible conditions. As shown in FIG. 2, for example, if the switching frequency is increased from fop (the operation frequency) to f1 under open lamp condition, the output voltage Vout will be the striking voltage Vstrike and high enough to ignite the lamp again if there is only one open lamp. But if there are two or more open lamps, the output voltage Vout will become much smaller than the striking voltage Vstrike.